High-bypass turbofan gas turbine engines include a nacelle surrounding a fan and defining a fan duct through which flows fan air. The air is compressed by the fan and a portion thereof is discharged from the fan duct for providing thrust for powering an aircraft in flight.
In order to reduce noise emanating from the fan, the fan duct is typically lined with acoustic panels. The panels typically include a honeycomb core sandwiched between a back sheet and a face sheet. The panels are typically light-weight structures which are easily crushable and, therefore, metallic bushings are typically provided through apertures in the panels for preventing fasteners extending therethrough from crushing the panels when mounted to the nacelle.
In order to prevent undesirable vibration of the panels during operation of the engine, the bushings must be suitably secured to the panels. The bushings may be bonded to the panels, or molded therein, or mechanically joined thereto, for example, depending on the particular structure of the panel. In one prior art design, the acoustic panel includes fiberglass laminated back and face sheets over an aluminum or Nomex (Trademark--aramid fiber) honeycomb core. Aluminum bushings are used and are preferably tapered to form a frusto-conical member which is received in a complementary tapered, frusto-conical aperture through the panel. The bushing and panel aperture are tapered so that during assembly of the bushings to the panels, the conventional potting adhesive placed within the panel aperture is not excessively ejected from the aperture when the bushing is inserted therein. For comparison purposes, a cylindrical bushing inserted into a cylindrical aperture containing the potting adhesive will more fully push the adhesive from the aperture when the bushing is inserted therein. The tapered aperture and bushing allows more of the adhesive to remain between the aperture and the bushing to ensure an acceptable adhesive bond around the entire bushing in the aperture of the panel. Upon curing, the bushing is fixedly joined to the panel, and a conventional fastener fixedly joins the bushing against the nacelle for fixedly supporting the acoustic panels in the fan duct.
However, sandwich panels having graphite-epoxy laminated back and face sheets are known to effect galvanic corrosion with aluminum. Accordingly, the relatively inexpensive aluminum bushings may not be used unless suitable protection against galvanic corrosion is provided. Titanium bushings could be used instead of aluminum without galvanic corrosion, but the cost and weight thereof is substantial.